Break In Engine won't hold compression

[Hell on Wheels]

I've been breaking in my engine after a rebuild and it isn't holding compression. It regains compression if I run alot of oil through it to where it makes a huge mess. Bad cylinder? Rings? It's not AFR because I can't get it to fire even with starter fluid.

 

[JerboaJohn]

back woods technique, pull the sparkplug, put thumb over hole and turn engine over. if there's no way you can keep the pressure from escaping past your thumb you have enough to fire it at least.

you installed a new piston and rings? anything else changed out for new? gaskets, seals etc,,

 

[mark20]

I've been breaking in my engine after a rebuild and it isn't holding compression. It regains compression if I run alot of oil through it to where it makes a huge mess. Bad cylinder? Rings? It's not AFR because I can't get it to fire even with starter fluid.

its a sign of worn rings (or a bad head gasket)
becuase the oil fills the gap worn, and inturn regains compression (known as a "wet" compression test)



good luck!

 

[Hell on Wheels]

back woods technique, pull the sparkplug, put thumb over hole and turn engine over. if there's no way you can keep the pressure from escaping past your thumb you have enough to fire it at least.

you installed a new piston and rings? anything else changed out for new? gaskets, seals etc,,

I replaced the cylinder, head gasket, rings, piston, and wrist pin and bearing

 

[FNTPuck]

Just to be clear, you didn't install a windowed piston without a reed did you? I have seen that done before and with a windowed piston you need a reed valve to control the airflow through the engine. Without it it won't start at all.

Also, if your piston is backwards it won't have enough compression to start. The locating pins for the piston rings should face the intake side - ignore the arrow, some are stamped incorrectly. With it backwards you will barely have 50-60psi and it either wont start or will sputter and have no power.

 

[Hell on Wheels]

I have a DIO reed and windowed piston.

The piston is installed correctly.

Maybe bad ring gap? Bad cylinder?

 

[Hell on Wheels]

Note, it ran like a charm once I threw some oil into the intake and more into my gas tank. Rode it 16 miles home (with some hicups like the pipe rattling off).

What should I look for when I take my cylinder out for exhaust reporting? I need alot more blowdown because it's cutting power @7000rpm when it should be pulling on the pipe way higher (I shortened the pipe when I changed the cylinder and it cuts quicker without the power).